1. Field
The present disclosure relates to an image processing filter, an image processing method of the image processing filter, and an image processing circuit of an image processing apparatus having the image processing filter.
2. Description of Related Art
In an image processing apparatus, for example, an image is read by an image sensor, and is averaged, so that the picture quality of the image is improved. In the image processing apparatus disclosed in Japanese Patent Publication No. 60-236580, the image being read by the image sensor is processed by a space filter to sharpen or smoothen the image, and thereby the image is improved.
In the image processing apparatus disclosed in Japanese Patent Publication No. 03-54679, an image signal obtained from a continuous gradation image is averaged according to the concentration and contrast data of the image signal, and noise is favorably reduced from the image signal.
In a recent digital camera, since the photo detection capability of an image pickup device has been improved, it is easier to detect random noise, as well as the light from the subject, and an S/N ratio is lowered. Accordingly, in the recent digital camera, it is demanded to lower random noise and enhance the S/N ratio without lowering the picture quality of the taken image.
Generally, the image processing apparatus for reducing random noise and improving the S/N ratio includes the space filter having weight table, a median filter, or an ε filter. The ε filter is characterized in that it sets the noise judging value ε in a specific range, and eliminates random noise of small amplification given to the signal while maintaining the abrupt large amplification change component of the signal.
In the image processing apparatus using the ε filter, if a relatively large noise is mixed into the target pixel, as shown in FIG. 14, the differential value between the pixel value of target pixel and the pixel value of surrounding pixel may exceed the noise judging value ε. In the image processing apparatus using the ε filter, in the case shown in FIG. 14, although noise is mixed into the true value of a target pixel, the pixel value containing the noise may be used when calculating the pixel value of the target pixel. Accordingly, in the image processing apparatus using the ε filter, due to the effect of the pixel value containing the noise, the pixel value of the target pixel may be prevented from approaching the true value of the target pixel.
Further, in the image processing apparatus using the ε filter, as shown in FIG. 15, the pixel value of target pixel may be present in the boundary (image boundary) of an image region of a specific color or an image region of other than specific color. In the image processing apparatus using the ε filter, if the pixel value of the target pixel is present near the image boundary, when calculating the pixel value of target pixel, for example, if an average value of two adjacent pixel values across the image boundary (an average pixel value) is used, the average pixel value may be closer to the pixel value adjacent to the average pixel value. Accordingly, in the image processing apparatus using the ε filter, when the average pixel value approaches the pixel value adjacent to the average pixel value, it is impossible to recognize the existence of the image boundary between the average pixel value and the pixel value adjacent to the average pixel value. As a result, in the image processing apparatus using the ε filter, the pixel value cannot be changed suddenly across the image boundary. Therefore, the image contrast gets lowered, and the image excellent in sharpness may not be obtained.